(1) Field of the Invention
The present invention relates to pneumatic radial tires. More particularly, the invention relates to pneumatic radial tires having high speed performances, flexural fatigue resistance and cornering stability improved.
(2) Related Art Statement
Since pneumatic radial tires have improved cornering stability, high speed durability and wear resistance as compared to conventional bias tires owing to the radial structure, the radial tires have become popularized.
In general, the radial tire has the structure in which a carcass having cords arranged in radial directions of the tire is held by a pair of beads and a belt surrounds the carcass. The belt consists of two or more cord layers having cords arranged at an angle of 10.degree. to 30.degree. with respect to a circumferential direction of the tire. The carcass is constituted by one or two plies having carcass cords arranged at substantially 90.degree. with respect to the circumferential direction.
The carcass and the belt have an important role to keep strength of the radial tire together with the bead portions. A primary feature of the radial tire lies in the belt and the carcass. The carcass affords flexibility upon the tire, whereas the belt restrains the carcass and functions as a hoop.
Tires having excellent high speed performance have been demanded due to the propagation of high speed roads, and it is necessity to develop new materials for the belts. Although steel cords are mainly used for belts in conventional radial tires, cords have the great problems for tires required for high speed use. That is, since a radially outer growth of the tire becomes greater by centrifugal forces during turning at high speeds due to heavy steel cords, movement inside the tire becomes greater and rolling loss becomes greater. Further, such rolling loss causes heat generation, which results in fracture of the tire. Therefore, steel cord belts are not suitable for tires required to have high speed performance.
On the other hand, research of organic fibers have recently been remarkably advanced. Particularly, light fiber materials having the possibility of replacing the steel cords have appeared. Such fiber materials are produced by crystal spinning, super-drawing, gel spinning or the like, and have high strength and high modulus of elasticity. For examples, they are aramid fibers, carbon fibers, super high tenacity polyethylene fibers, high strength PVA fibers, polyoxymethylene fibers or whole aromatic polyester fibers. These fibers are about one fifth as heavy as the weight of the steel cords, but have strength and modulus of elasticity comparable to those of the steel cords.
However, the organic fibers are each constituted by a number of fine filaments. Therefore, when the organic fibers are used in the state that they are twisted and bundled same manner as in the conventional tire cords made of organic fibers, the modulus of elasticity in tension is lowered owing to twisting. Furthermore, since flexural rigidity and compression rigidity are low due to flexibility of fine filaments, rigidity of the belt in the radial tire decreases. Because of this, sufficient belt rigidity cannot be obtained unlike the case of the steel cords, so that cornering stability, high speed durability and wear resistance are deteriorated.